This project is concerned with the regulation of metabolism in preimplantation mouse embryos. The overall objective is to define the mechanism by which the mother can render delayed implanting embryos dormant, maintain them in that condition for several days, and then cause them to resume development. Experiments will be conducted at three different levels in an attempt to define point at which the control processes function. Synthesis of RNA. The relative changes in synthesis of mRNA will be determined as delayed implanting embryos are activated in vivo and in vitro. The patter will be compared with that of rRNA as well as overall RNA synthesis to determine whether an increase in synthesis of mRNA precedes that of the others. Messenger RNA will be isolated by means of chromatography with oligo(dT) cellulose. Puromycin and cyclohexamide will be used to determine whether the increases in RNA require prior synthesis of protein. Interaction of the Inner Cell Mass (ICM) and Trophoblast Cells. It has been generally assumed that the ICM influences cell division in trophoblast cells. The ICM will be removed, or a second one added, to delayed implanting embryos by microsurgery. The embryos will be activated with estradiol-17Beta and the effects of the ICM on DNA synthesis will be determined by autoradiography. In vitro Activation of Delayed Implanting Embryos. Qualitative aspects of RNA synthesis and protein synthesis will be compared as embryos are activated in vivo an in vitro. This will be done in order to determine whether the process in vitro is the same as that in vivo and evaluate its usefulness as an experimental model. The effect of low concentrations of Alpha-amanitin and actinomycin D on the synthesis of specific proteins during activation wil be examined.